Moria is literally irrelevant in this situation. You have no proof of celestials being unaware of his death and the Gorosei change, for all we know they are familiar with both Saturn and Garling. Goroseis even give audience to random people, kings or influential figures like Vegapunk or Shanks. Of course the Holy Knights will also know, just like any relevant Marine figure. The best Devon can do in that regard is enter a fodder base like Navarone and pull a Condoriano 2.0.
Chances are she could use his authority on Seraphim though, assuming they don't just swap his name with Garling's.
They have no reason to swap his name if they think he is dead. They will just add Figarland as extra.
So Saturn being dead instead of left alive with random Celestial rights is better if they left Saturn alive them they would have to swap or remove Saturn's authority over Seraphims. Since now they think he is dead, they won't bother to remove or swap.
The point that you failed to understand about Moria example is that Government lie about things. Its not shocking if they lied about Saturn's death if they see him alive. They didn't know Devon copied him, if they see Saturn again they will just think somehow Saturn survived and since real Saturn isn't alive to expose the disguise it will be difficult for them to understand whats going on.
Even Scabbards almost got fooled by Kanjuro's Oden trick, Asura died for that. They literally see Oden boiled to death and they left him there, still they thought Oden was alive and you argue other Celestials and Admirals won't believe Saturn somehow survived? Lmao.